vmalert: fix vmalert_remotewrite_send_duration_seconds_total metric…

… value (#4801)

The deferred call's arguments are evaluated immediately, but the function call is not executed until the surrounding function returns.

Signed-off-by: Abirdcfly <fp544037857@gmail.com>

app/vmalert/remotewrite/remotewrite.go

@@ -216,7 +216,9 @@ func (c *Client) flush(ctx context.Context, wr *prompbmarshal.WriteRequest) {
 		retryInterval = maxRetryInterval
 	}
 	timeStart := time.Now()
-	defer sendDuration.Add(time.Since(timeStart).Seconds())
+	defer func() {
+		sendDuration.Add(time.Since(timeStart).Seconds())
+	}()
 L:
 	for attempts := 0; ; attempts++ {
 		err := c.send(ctx, b)